Sputter targets made of ferromagnetic materials are critical to thin film deposition in industries such as data storage and VLSI (very large scale integration)/semiconductors. Magnetron cathode sputtering is one means of sputtering magnetic thin films.
The cathode sputtering process involves ion bombardment of a target composed of a ferromagnetic material. The target forms part of a cathode assembly in an evacuated chamber containing an inert gas, such as argon. An electric field is applied between the cathode assembly and an anode in the chamber, and the gas is ionized by collision with electrons ejected from the surface of the cathode, forming a plasma between the target surface and the substrate. The positive gas ions are attracted to the cathode surface, and particles of material dislodged when the ions strike the target then traverse the enclosure and deposit as a thin film onto a substrate or substrates positioned on a support maintained at or near anode potential.
Although the sputtering process can be carried out solely in an electric field, substantially increased deposition rates are possible with magnetron cathode sputtering, in which an arched magnetic field, formed in a closed loop over the surface of the sputter target, is superimposed on the electric field. The arched closed-loop magnetic field traps electrons in an annular region adjacent to the surface of the target, thereby multiplying the collisions between electrons and gas atoms to produce a corresponding increase in the number of ions in that region. The magnetic field is typically created by placing one or more magnets behind the target. This produces a leakage magnetic field on the surface of the target so that the plasma density may be increased.
Erosion of particles from the sputter target surface generally occurs in a relatively narrow ring-shaped region corresponding to the shape of the closed-loop magnetic field. Only the portion of the total target material in this erosion groove, the so-called "race track" region, is consumed before the target must be replaced. The result is that typically only 18-25% of the target material is utilized. Thus, a considerable amount of material, which is generally very expensive, is either wasted or must be recycled. Furthermore, a considerable amount of deposition equipment "down-time" occurs due to the necessity of frequent target replacement.
To solve these disadvantages of the magnetron sputtering process, various possible solutions have been pursued. One potential solution is to increase the thickness of the target. If the target is relatively thick, then sputtering can proceed for a longer period of time before the race track region is consumed. Ferromagnetic materials, however, present a difficulty not encountered with non-ferromagnetic materials. For magnetron sputtering, the magnetic leakage flux (MLF) or leakage magnetic field at the target surface must be high enough to start and sustain the plasma. Under normal sputtering conditions, such as an argon pressure of 5-10 mTorr, the minimum MLF, also known as pass through flux (PTF), is approximately 150 gauss at the sputtering surface, and preferably is about 200 gauss for high speed sputtering. The cathode magnet strength in part determines the MLF. The higher the magnet strength, the higher the MLF. In the case of ferromagnetic sputter targets, however, the high intrinsic magnetic permeability of the material effectively shields or shunts the magnetic field from the magnets behind the target and hence reduces the MLF on the target surface.
For air and non-ferromagnetic materials, permeability is very close to 1.0. Ferromagnetic materials, as referred to herein, are those materials having an intrinsic magnetic permeability greater than 1.0. Magnetic permeability describes the response (magnetization) of a material under a magnetic field. In CGS units, it is defined as: EQU Permeability=1+4.pi.(M/H)
where M is the magnetization and H is the magnetic field. For currently available sputter targets, the permeability ranges from close to 1.0 to 100 or higher. The value depends on the particular material and manufacturing process. For example, a machined Co sputter target has a permeability throughout of less than about 10, whereas machined NiFe and Fe sputter targets have permeabilities throughout of greater than 20.
Because of high permeability and thus low MLF, and because MLF decreases with increasing distance from the cathode magnets, ferromagnetic sputter targets are generally made much thinner than non-magnetic sputter targets to allow enough magnetic field to be leaked out to the sputtering surface to sustain the sputtering plasma necessary for magnetron sputtering. Non-ferromagnetic targets are typically 0.25 inch thick or greater, whereas ferromagnetic targets are generally less than 0.25 inch thick. With some ferromagnetic materials, particularly those with higher permeability, the targets have to be machined to 0.0625 inch thick or less to achieve an MLF at the sputtering surface of 150 gauss, and some very high permeability materials are impossible to magnetron sputter because an MLF of 150 gauss simply cannot be achieved. Thus, not only can these ferromagnetic targets not simply be made thicker so as to reduce equipment down-time, they must actually be made thinner. To increase thickness, the MLF must somehow be increased.
U.S. Pat. No. 4,401,546 discloses a planar ferromagnetic target that achieves a thickness of 0.20 inch (5 mm) by means of a segmented target, where the segments are separated by gaps through which the magnetic field leaks to produce an MLF of 200 gauss on the surface of the target. This is described as being an improvement over conventional ferromagnetic targets that could be machined to no thicker than 0.055 inch (1.4 mm), preferably no thicker than 0.028 inch (0.7 mm), to produce an MLF of 200 gauss.
U.S. Pat. No. 4,412,907 also discloses, in the embodiment of FIG. 4, a segmented planar ferromagnetic target up to 1 inch thick (25 mm) with individual segments having sloped portions so as to produce angled gaps through which the magnetic field leaks to produce an MLF at the surface of the target of up to 730 gauss.
U.S. Pat. No. 5,827,414 discloses a planar ferromagnetic target that claims to achieve a thickness of 0.16-1.0 inch (4-25 mm), also by gaps in the target. The gaps in this configuration are radial gaps formed by slots in the target body that are perpendicular to the flux of the magnetron, thereby producing a more effective and homogeneous leakage magnetic field on and parallel to the surface of the target body so that the sputtering plasma density may be increased.
These methods of machining slots or grooves into the target body and assembling a target from individual segments so as to increase the MLF, although allowing for thicker targets, are undesirable because the gaps allow for the sputtering and deposition of foreign particles, such as from the backing plate, or severe particle generation due to the accumulation of target redeposition materials in the slotted or grooved regions. This particle generation in the thin films sacrifices the integrity of the article being coated. Furthermore, this solution of increasing target thickness by removing target material along certain patterns like grooves or slots does nothing to decrease the amount of target material waste.
In general, the higher the permeability of the ferromagnetic material, the thinner the sputter target is required to be. Such a limitation on target thickness, however, leads to a shorter target life, waste of material and a need for more frequent target replacement. Furthermore, the high permeability and low MLF of a ferromagnetic target can cause problems of high impedance, low deposition rates, narrow erosion grooves, poor film uniformity and poor film performance. It is thus desirable to provide a high MLF ferromagnetic sputter target that may be made relatively thick without sacrificing film integrity.